1. Field of the Invention
This invention generally relates to a circuit for and method of processing periodic electrical signals and, more specifically, for an improved comb filtering technique for separating the luminance from the chrominance information contained in a color television composite video signal.
2. Description of the Prior Art
In color television systems, the intensity and color information which is produced by scanning a scene at a periodic horizontal scanning frequency, such as in the NTSC system developed in the United States, the monochrome intensity, or luminance, information and the color, or chrominance, information are interleaved in order to compress or minimize the total video bandwidth required to transmit both. With this arrangement, the luminance components, considered in the frequency domain, are clustered at integral multiples of the horizontal line scanning frequency (15,750 Hz) and the chrominance components are clustered at odd multiples of 1/2 the line scanning frequency. The major portion of the luminance energy is contained in low video frequencies contained in a bandwidth of approximately 2 MHz out of the total of the approximate 4.2 MHz bandwidth available to transmit picture information. While the major portion of the luminance energy is primarily contained within the lower frequencies of the available bandwidth, most of the color or chrominance information is contained within the upper 2 MHz of that video bandwidth. With older receivers, the luminance and chrominance signals were separated by simply using band pass filters to respectively transmit the desired and attenuate the undesired upper and lower frequency portions of the video spectrum, with the lower portion being directed to the luminance channel, and the upper portion being directed into the chrominance channel for further processing. A common approach is to limit the luminance (Y) frequencies to about 3.2 MHz. However, the high luminance frequencies enhance the horizontal (fine detail) resolution. Because the luminance components generally trail off in amplitude in the chrominance portion of the video spectrum, the luminance signals when processed in the chrominance channel do not unduly interfere with color processing although some interference could be detected. Likewise, the chrominance signals progressively decrease in amplitude in the lower luminance frequency portion of the video bandwidth but could produce some minor interference in the luminance channel. Such interference by each signal in the processing channel of the other signal is normally acceptable and such approach is widely used although the interference represents a deterioration of picture quality and the picture raster clarity or sharpness is less than optimum.
More recently, with the availability of low cost comb filters and delay lines, more and more receivers utilize comb filtering techniques for separating the luminance from the chrominance signals. Various comb filter arrangements for this purpose have been proposed. For example, such filtering techniques are described in U.S. Pat. Nos. 4,096,516 and 4,074,321. A problem that has been encountered, however, in the use of broadband combing techniques has been the loss of "vertical detail" in the picture. As is well known, extension of combing action into the low frequency luminance portion of the video bandwidth, which is not shared with the chrominance signal components, results in a drop off of the low frequency luminance frequency components. The luminance information has the same phase and is basically unchanged from scanning line to scanning line. The chrominance information, however, has its phase reversed from one scanning line to the next although the information content is otherwise the same. The comb filter will reject, for example, those frequencies (luminance) which have the same phase from line-to-line and pass with double amplitude those frequencies (chrominance) which have the opposite phases from line-to-line. The comb filter, can also be configurated to double the luminance frequencies while rejecting the chrominance frequencies. Since low frequency video signals represent the "vertical details," it is important to preserve the "flatness" of the low frequency video response in order to avoid loss of vertical resolution and, therefore, the integrity of all elements of the displayed image.
As a result of the loss of vertical detail information encountered with broad band combing, two approaches have been adopted. The first limits the combing action to the bandwidth of approximately 3-4 MHz in order to avoid combing of the low video frequencies with attendant distortion thereof. Another approach, when broad band combing has been performed, is to compensate for the loss of vertical details by using elaborate vertical detail enhancement circuitry, the purpose of which is to restore the lost low frequency video information. However, unless compensation is essentially in the correct amount, under compensation or over compensation may still result in a loss of vertical resolution and the extra cost of this circuitry does not always resolve the problem. Therefore, most comb filter structures result in a compromise between improved separation of the luminance and chrominance signals from each other and, consequently, "cleaner" signals in each of the luminance and chrominance channels and loss of vertical definition. The use of a comb filter provides approximately 25% increase in the horizontal (fine detail) resolution.